There are a variety of applications where canceling inductive reactance over a wide frequency bands are required. Such applications can benefit from negative inductance circuits to offset the inductive reactance and provide for efficient power transfers. Examples of such applications include systems with small transmit antennas, and in particular to antennas at and below VHF frequencies. In order to operate efficiently, antennas must be matched to the transmitting circuitry.
Large antennas are more easily matched because they operate at a relatively low Q factor. However, full size antennas are often times not feasible on mobile platforms such as aircraft. This necessitates use of compact antennas with their associated high Q.
One approach to providing matching for compact antennas is to resonate the load with capacitance. The disadvantage is that it results in an extremely narrowband performance for high-Q loads, such as compact antennas, and is not useful if wideband performance is required. In addition, performance outside the band of interest results in extremely poor performance.
Another approach to drive an antenna is either unmatched or with lossy matching. This approach has extremely low power efficiency. Large and expensive power amplifiers and cooling systems are needed to generate and dissipate RF power. The unmatched case complicates amplifier design because it must handle reflected power, while lossy matching further reduces efficiency.
The use of small signal non-Foster circuits to match antennas has been tried. Such use does not extend to high power levels due to their high power dissipation, leading to low efficiency.
The use of resonant non-Foster circuits overcomes the high voltage problem by resonating the load then canceling the reactance with a resonant non-Foster circuit. At the resonant frequency, the voltage across the NFC is zero; however, this voltage quickly rises above and below the resonant frequency. Therefore, the high power efficiency is only realized over a bandwidth comparable to passive matching.
Waveform synthesis methods do not attempt to resonate the antenna but instead simply drive it with an on-off waveform. The two main disadvantages are a) it requires digital synthesis control and does not respond to an input waveform and b) the antenna is not resonated and must essentially be driven by a voltage source. This gives no flexibility to obtain the desired frequency response.
Linear non-Foster circuits (NFCs) are based on the concept of impedance and are well known. Current negative inductance circuits, often referred to as non-Foster circuits because they break Foster's reactance theorem, use amplifiers, transistors operating in small signal transconductance mode or negative resistances to generate a small-signal impedance approximating jωL, where L is negative. These circuits are typically biased in class A, meaning that they constantly draw DC current and dissipate DC power.
The current small signal circuits do not extend to the large signal regime applicable to transmit antennas because of the high-voltage problem. Consider power delivery to a reactive load with impedance Z=R+jX with X>>R and a quality factor Q=X/R. The power delivered to the load is I2*R and the magnitude of the voltage across the load is approximately 1*X. One may employ an NFC to “resonate” the load over a wide bandwidth, giving an input impedance Z′=R+jX−jX=R. Now the current flowing through the NFC is I, resulting in a voltage of I*X. The active devices must be biased with enough headroom to handle both the current I and the voltage I*X. This means that while the NFC dissipates no RF power, the DC power dissipation is I2*X, which is Q times the RF power delivered to the load. Resonant NFCs have been proposed to solve this problem, but the power efficiency is obtainable only over a narrowband.
In short, passive networks can only match an antenna for discrete frequencies and resonating loads with capacitance leads to narrowband responses. NFC circuits are either inherently narrow band or dissipate proportionally high DC power, hence less efficient.
Hence, there is a compelling need in the field of circuit synthesis for simple and efficient negative inductance circuits that can operate at high power levels with high efficiency over a wide frequency band. Such circuits will allow the use of compact antennas for wideband applications.